In a large-capacity power conversion device, since the converter output has high voltage or large current, a configuration having a plurality of converters in series or parallel in a multiplexed manner is often used. It is known that multiplexing a converter not only increases a converter capacity, but also reduces a harmonic contained in an output voltage waveform by synthesizing outputs, and as a result, can reduce harmonic current flowing into a system.
There are various methods for multiplexing a converter: reactor multiplexing, transformer multiplexing, direct multiplexing, etc. In the case of transformer multiplexing, since an AC side is isolated by transformers, there is an advantage that common DC voltage can be used among the transformers. However, there is a disadvantage that, in the case where output voltage is high, the configuration of the multiplexed transformer is complicated and the cost of the transformer increases.
Considering the above, as a power conversion device that is suitable for high-voltage usage and does not require a multiplexed transformer, a multilevel converter is proposed in which outputs of a plurality of converters are connected in cascade. One example of such multilevel converters is a modular multilevel converter (hereinafter, referred to as an MMC).
The MMC is composed of an arm in which a plurality of unit converters called cells (hereinafter, referred to as unit cells) are connected in cascade. Each unit cell includes a plurality of semiconductor switches and a DC capacitor, and through ON/OFF control of the semiconductor switches, outputs both-end voltage of the DC capacitor and zero voltage.
In the case of three-phase MMC, such an arm is individually formed for each phase, an output end of a half number of cells of total cells connected in cascade is used as an AC terminal, both ends of the arms for respective phases are connected to each other, and their respective terminals are used as DC terminals. Since each unit cell output of the MMC is connected to both sides of the AC end and the DC end of the MMC, each unit cell has a characteristic of outputting both DC current and AC current. That is, current flowing in each arm includes an AC component and a DC component. Therefore, in the MMC, it is necessary to control these plural current components. Examples of such control methods are shown in Patent Documents 1 and 2 and Non-Patent Document 1.
These documents disclose that, in an MMC having a DC side connected to a DC power supply and an AC side connected to an AC power supply, AC current is controlled on the AC side of the MMC, and current obtained by removing a component relevant to AC output from each arm current is controlled on the DC side, thus attaining the above control.